1. Field of the Invention
The present invention relates to an electrostatic discharge protection device that protects an electronic device from damage caused by electrostatic discharge, and to a method of manufacturing the same.
2. Description of the Related Art
Electrostatic discharge protection devices (ESD protection devices) are broadly used to prevent damage, erroneous operations, and the like in electronic devices caused by electrostatic discharge (ESD).
For example, International Publication WO 2008/146514 discloses an ESD protection device having a multilayer ceramic substrate, a hollow cavity portion formed within the multilayer ceramic substrate, at least one pair of discharge electrodes having opposing portions arranged so that tip portions thereof oppose each other with an interval within the hollow cavity portion, and an outer electrode that is formed on a surface of the multilayer ceramic substrate and is connected to the discharge electrodes. According to the ESD protection device disclosed in International Publication WO 2008/146514, the multilayer ceramic substrate includes a mixed portion containing a metal material and a ceramic material, the mixed portion being disposed near the surface where the discharge electrodes are provided and adjacent to at least an opposing portion and a part that is between the opposing portions of the discharge electrodes. The content of the metal material in the mixed portion is greater than or equal to 10 vol % and less than or equal to 50 vol %. By having the above-described configuration, the ESD protection device disclosed in International Publication WO 2008/146514 can accurately set a discharge start voltage.
International Publication WO 2013/011821 discloses an ESD protection device including first and second discharge electrodes opposing each other, a discharge auxiliary electrode extending across the first and second discharge electrodes, and an insulating substrate that holds the first and second discharge electrodes along with the discharge auxiliary electrode. In the ESD protection device disclosed in International Publication WO 2013/011821, the discharge auxiliary electrode includes a combination of a plurality of metal particles including a core-shell structure including a core portion that has a first metal as its primary component and a shell portion that has a metal oxide containing a second metal as its primary component. By having the above-described configuration, the ESD protection device disclosed in International Publication WO 2013/011821 provides high insulation reliability and favorable discharge characteristics.
Japanese Unexamined Patent Application Publication No. 2008-85284 discloses an overvoltage protection element including a first electrode, a second electrode, and a porous structure, connected between the first electrode and the second electrode, that is produced by carrying out a firing process using materials of the overvoltage protection element, which include a non-conductive powder, a metal conductive powder, and a bonding agent. Using the above-described materials of the overvoltage protection element when manufacturing the overvoltage protection element disclosed in Japanese Unexamined Patent Application Publication No. 2008-85284 makes it possible to significantly reduce or prevent damage caused by discharged energy, which in turn makes it possible to extend the usage lifespan of the element.
Recently, as electronic devices continue to have higher levels of performance, there is demand for lower discharge start voltages in ESD protection devices for the purpose of more reliably preventing damage, erroneous operations, and the like in electronic devices caused by electrostatic discharge. On the other hand, there is also demand for ESD protection devices to have insulation properties (initial insulation properties) high enough to significantly reduce or prevent the occurrence of short-circuit in normal usage situations. There is also a demand for the development of ESD protection devices having superior repetition resistance, or in other words, that are capable of suppressing drops in insulation properties even under the repeated application of ESD.